Pit bull Ace was the main “demonstration dog” at Marlon Grennan’s protection dog training company Dark Dynasty K9s (DDK9s), famous for his acrobatic leaps through car windows and his ferocious drive.

He was attacked by two much larger, younger dogs on June 26, 2018, at the company’s New Hampshire HQ after accidentally being let out at the same time as both of them and, crucially, a lone female.

The tragic story is featured in the new season of Barcroft TV’s hit online show Dog Dynasty, which has racked up more than 150 million views on YouTube and Facebook.

The show follows the fortunes of Hulk and the other DDK9 protection dogs which sell for between $5000 and $25,000 and has catapulted Hulk to global superstardom, with the loveable giant having his own dog food brand and merchandise line.

DDK9s founder Marlon Grennan told Barcroft TV: “It was just the worst possible situation: one female dog and three male dogs coming out of heat.

“You are talking about 300 pounds of dogs versus a dog that is 80 pounds, he didn’t stand a chance."

In the battle for the female, Ace was badly wounded and found by owners Marlon, 31, and Lisa, 27, covered from head to toe in mud.

The pair took him to the vet where he was stabilised and sent home - but later that evening his condition unexpectedly deteriorated, and the family was forced to rush him back to the emergency vets.

Lisa said: “Ace was a little shaky so I went to the truck and got the blanket and we wrapped him up.

“Marlon was doing everything he possibly could - CPR, holding the tubes up - but he had gone into cardiac arrest.

“There was nothing that could be done and I just stroked his face, I told him I loved him and I just held him until he stopped."

Ace’s passion for training made him a star, with online viewers frequently mistaking his incredible athleticism for giant pit bull Hulk.

Marlon said: "Ace really was an incredible dog, I just never really thought it would end like this.

“He always showed himself from day one, from the beginning he stood out like a shining star.

“He was like my son - this dog was me in dog form. It was my mistake and human beings make mistakes and we are not perfect but I’m not sure I can ever really forgive myself.

“The two dogs didn’t do anything wrong, they were just reacting as animals react in this situation - it was my fault for allowing them to be in that situation, as a leader I failed them."

Lisa, who birthed Ace, said: “He was a crazy little puppy, that’s why we chose him to do protection work.

"Out of the whole litter he stood out the most, he was incredible, all he wanted to do was bite and he’d be latched onto your pant leg.”

In order to say goodbye to Ace as a family, Marlon and Lisa decided to visit a nearby ‘dog chapel’ in St Johnsbury, Vermont with their children Jordan, 7, and Jackson, 2.

The chapel is situated on Dog Mountain, a 150-acre mountaintop retreat set up by the artist and children’s author Stephen Huneck and his wife Gwen, to celebrate the connection between man and his best friend.

Marlon said: “Going to the dog chapel was pretty powerful. As soon as I opened the door all those people’s dogs were surging through my body and all these things these people had written like this dome of sadness. But at the same time, people’s forever love for their animals.

“There will never be another Ace, he’s a living legend and I’m going to get him tattooed on me.

“His death was a tragedy, but it’s made me go back to basics, back to what brought us fame, which is my love and passion for dogs and dog training.

“Ace struck fear in a lot of people because to them he didn’t seem as cuddly - but he was the sweetest dog ever.”

Dog Dynasty season 3 is available on Barcroft Animals YouTube page and the Dog Dynasty Facebook Watch page.